This invention generally relates to metal-based coatings to inhibit metal catalyzed coke formation on metal alloy surfaces of process equipment used in hydrocarbon conversion processes.
Hydrocarbon conversion processes typically require reactor systems, and associated conduits and piping, adapted for hydrocracking, reforming, fluid catalytic cracking, and other similar processes. At the process conditions and temperatures that are required for hydrocarbon conversion, solid carbonaceous materials, referred to as coke, typically form on the metal alloy surfaces of the reactor components and associated equipment due to metal catalyzed reactions at the metal alloy surfaces. The formation of metal catalyzed coke deposits is influenced by factors such as the content of the hydrocarbon feed, the conversion process, the specific reaction conditions, and the material and configuration of the reactor and associated equipment.
For many hydrocarbon processes in refining and petrochemical services, as typically employed, metal catalyzed coke deposits commonly occur at appreciable rates from about 350° C. (662° F.) to about 850° C. (1562° F.). The formation of metal catalyzed coke deposits for a particular process is dependent upon the process conditions, the composition of the hydrocarbon stream, the compositions of the metal surfaces in contact with the hydrocarbon stream, the amount of time that the metal and hydrocarbon stream are in contact, and other similar considerations. The temperature at which a process will produce metal catalyzed coke can be referred to as the metal catalyzed coke onset temperature. If a process is operated at temperatures greater than the coke onset temperature, the build up of metal catalyzed coke deposits can cause a number of significant problems within a reactor and associated equipment, and may result in severe heat transfer reductions, undesirable pressure drops within the process, loss of process efficiencies, and premature shut down of the conversion process.
Excess metal catalyzed coke build-up, for example, can cover catalyst sites, plug catalyst pores, and clog catalyst screens retaining the catalyst within the reactor. Such excess deposits also can build-up on other reactor internal components and accumulate in the piping and passages within the reactor and associated equipment. Thus, such deposits can reduce catalyst activity, interfere with the efficient transfer of heat through heated or heat exchanging surfaces, and can create significant product flow reductions. Accordingly, metal catalyzed coke deposits can become sufficiently severe to require the premature shut down of the hydrocarbon conversion process to regenerate the catalyst, and to decoke and replace reactor and other surfaces subject to the coke deposits, as well as those damaged components by coking and associated reactions.
Metals that catalyze the coke formation may reside on or in the catalyst, may come from the process equipment, or can exist in the feed stream. Typical catalyzing metals include: Mn, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, and Cu, among others. Such metals and their alloys are frequently used in hydrocarbon conversion reactors and related equipment and therefore often provide reaction sites for metal catalyzed coke formation. For instance, where a reactor or related components are austenitic stainless steel 300-series, the iron, chromium, and/or nickel metal alloys of the stainless steel provide the sites and surfaces for precipitates of metal catalyzed coke at the operating conditions and temperatures used for hydrocarbon conversion processes.
Use of process temperatures below metal catalyzed coke onset temperatures, may help reduce or eliminate metal catalyzed coke formation, but the use of such reduced temperatures often is not desirable. Many hydrocarbon conversion processes require or benefit from temperatures approaching or exceeding the coke onset temperature to obtain efficient conversion rates and desired hydrocarbon conversion products and product mixes. Often methods for reducing coke formation, such as using antifoulant additives, applying coatings to the inside of the process equipment or combining the metal alloys of the process equipment with an anti-coking agent have met with limited success.
In some instances, sulfur or sulfur compositions are used to inhibit metal catalyzed coke formation at the metal alloy surfaces of reactors and associated equipment. The introduction of sulfur can raise the coke on-set temperature and thus prevents metal catalyzed coke at the operating temperature by increasing the coke on-set temperature to above the operating temperature, however, sulfidation attack of the metal then can become a concern. Such sulfur compositions used as metal catalyzed coking inhibitors typically are introduced by addition to the initial hydrocarbon stream and may be added during the conversion process. Where the sulfur inhibitors are provided by addition, they typically are supplied by the addition of dimethyl sulfide (C2H6S) or dimethyldisulfide (C2H6S2) to the process stream, which then form hydrogen sulfide (H2S) in the process stream. In such approaches, the typical concentration levels of H2S typically are from about 0.25 wt. ppm to about 200 wt. ppm. At temperatures above about 350° C. (662° F.), typical reactor materials, such as type 347 stainless steel, are corrosively reactive with H2S in the hydrocarbon stream as a result of sulfidation reactions between the H2S and the metals of the metal alloy surfaces. Such corrosion of the metal alloy surfaces can substantially reduce the useful life of the reactor components and related equipment, cause fouling, and interfere with the operation of the hydrocarbon conversion process.
The corrosion rates due to such sulfidation reactions, in some instances, may be relatively low, such as about 1 to about 10 mils per year (mpy). However, certain reactor components, such as catalyst screens and other similar relatively tight tolerance components, nevertheless are adversely affected by such corrosion rates. For example, the catalyst screens used for catalyst containment in many reactors typically have very small slots or spaces, e.g., less than 1.58 mm ( 1/16 inch) through which a feed stream can pass without carrying the catalyst out of the containment area. Even relatively low corrosion rates can corrode the screens to the point where the slots in the screens are sufficiently enlarged to permit catalyst to escape from behind the screens into other parts of the reactor system.
In some instances, control of the corrosion rate due to the sulfidation attack of the metal alloy surfaces is maintained by providing a sulfur partial pressure that produces a scale or layer of CrS on the metal alloy surfaces to minimize the diffusion of iron (Fe) from the metal alloy to the reactive surface and thereby reduce the available Fe susceptible to sulfidation corrosion. The equipment used in such approaches, however, must contain metal alloys having more than about 15% by weight chromium (Cr) in order to produce a sufficiently continuous Cr-type scale under the typical process conditions. Corrosion rates due to sulfidation may be less for metal alloys with higher levels of Cr than the austenitic stainless steels, but these alloys tend to be quite expensive or are not feasible because of availability, formability or other mechanical property issues.
The majority of the processes that use H2S to mitigate metal catalyzed coke formation, in addition, do not use process components that contain such high amounts of chromium, such as austenitic stainless steel materials. Where the metal alloy used has lower chromium content, the use of H2S tends to produce an outer FeS scale on the alloy surfaces that is friable under thermal stresses or catalyst transport through the system. Such friable surfaces can crack or erode causing further corrosion as fresh metal underneath the scale is exposed to a corrosive atmosphere. Enhanced corrosion may occur where Fe diffuses rapidly through the scale to form FeS scale that also can flake off very readily and expose the metal beneath it to more corrosion. Such scale flakes may foul the reactor components causing pressure buildup, loss in reactor efficiency and other such difficulties, and also may ultimately force the premature shutdown of the reactor. Depending on the sulfidation conditions, the inner Cr-rich sulfur scale may have poor adhesion and can flake from the reactor components.
Another approach for processes utilizing sulfur compositions as metal catalyzed coking inhibitors is to fabricate the process equipment from metal alloys rich in aluminum (Al) or silicon (Si). Upon exposure to elevated air temperatures, such alloys produce an outer scale of corrosion resistant Al2O3 or SiO2. For example, materials such as Haynes Alloy 214 manufactured by Haynes International, Inc. in Kokomo, Ind., evidenced reduced corrosion rates in such processes at coke onset temperatures, but such materials are expensive, difficult to fabricate and are not easily welded.
Some hydrocarbon conversion processes also operate under conditions where the formation of CrS scale on the exposed metal alloy surfaces does not sufficiently reduce the migration of Fe to the exposed surfaces, resulting in excess and deleterious FeS scale formation. The use of a more corrosion resistant alloy often is not feasible under such operating conditions. Under such reaction conditions, in addition, coke can build up on the catalyst over time, requiring the continuous removal of the partially deactivated catalysts from the bottom of the reactor stack for regeneration ex-situ, i.e., not within the reactor. Furthermore, the C5 and C6 content of the feed stream often must be limited and/or treatment of the feed stream carried out to eliminate or minimize coke precursors to reduce the rate of coke formation in the process.
There further are some hydrocarbon conversion processes where sulfur and sulfur composition inhibitors cannot be utilized for metal catalyzed coke reduction. Such processes include those utilizing a platinum-containing catalyst that is susceptible to sulfur poisoning. In such processes, the hydrocarbon feed must have sulfur concentrations less than about 0.1 ppm. To obtain improved performance and process efficiencies, a higher operating temperature often is desired, but the use of such increased temperatures above the threshold for metal catalyzed coke formation results in deleterious coke deposits on the catalysts, reactor components and other related components.
Applying coatings to the exposed surfaces of the equipment for such conversion processes to reduce metal catalyzed coke formation has not proved satisfactory for a number of reasons. For example, coatings tested under conditions comprising normal hexane, 78% H2 and no H2S at a pressure of about 414 kPa (60 psig) and a temperature of about 560° C. (1040° F.) often failed because the coatings delaminated and/or the coatings did not prevent metal catalyzed coke formation. In another approach, a tin (Sn) coating that formed a Fe—Sn intermetallic coating on reactor metal surfaces requiring a sulfur-free environment was used. The coating, however, provided little protection from H2S corrosion and, at times, would react with any H2S content in the feed stream. A Sn coating also risked poisoning the catalyst in some processes, and the Sn coating could not be used under catalyst regeneration conditions.
In another approach, an aluminum coating may be added to the reactor components by powder and vapor diffusion processes (e.g., Alonizing), such that an aluminum diffusion coating is created. Such a high temperature diffusion process alloys or diffuses Al into the surface of the metal at temperatures above about 800° C. (1472° F.) in order to obtain the appropriate diffusion layer. Such aluminum coatings in combination with steam and CO2 are believed to provide some resistance to coking in certain hydrocarbon processes, such as thermal cracking of hydrocarbon feedstocks. Similarly, a reactor can be used where its metal alloy components already incorporates the Al, and therefore exhibits similar resistance to coking.
Such aluminum coated or aluminum-containing metal systems typically require the addition of carbon dioxide and steam into the process in an effort to oxidize the Al coating/metal components to provide a coke resistant surface. The metal surfaces normally must contain nickel or cobalt, or both, to support the surface oxidized Al, and this is not effective in reactor systems made of other materials. This approach further resulted in increased operating costs due to the addition of the necessary carbon dioxide and steam. A further concern of such approaches for reducing coke formation is that certain hydrocarbon processes contain feed streams that must be processed under reducing conditions, such that CO2 or CO or other oxidants cannot be allowed in the process.
Such aluminum coated metal alloy surfaces or the aluminum-containing alloy, in addition, may not be resistant to coking, and actually may promote a substantial amount of coking at elevated temperature conditions, for example at temperatures greater than about 427° C. (800° F.). The high temperatures required to apply the aluminum coating onto a reactor by alonizing or other similar techniques also can warp or otherwise damage the reactor components.